This invention relates generally to improvements in measurement methods and apparatus and, more particularly, to a new and improved electronic thermometer system enabling very rapid, accurate, reliable and easily read temperature measurements.
It is common practice in the medical arts, as in hospitals and in doctors' offices, to measure the body temperature of a patient by means of a glass bulb thermometer incorporating a heat responsive mercury column which expands and contracts a calibrated temperature scale. Typically, the glass thermometer is inserted into the patient, either orally or rectally, and subsequently removed after a sufficient time interval has passed to enable the temperature of the thermometer to stabilize at the body temperature of the patient. This time interval is usually of the order of two to four minutes. After a sufficient period of time has passed, the thermometer is removed from the patient and is subsequently read by appropriate medical personnel.
It will be apparent from the foregoing that conventional temperature measurement procedures using glass bulb thermometers and the like are prone to a number of significant deficiencies. Temperature measurement is rather slow and, for patients who cannot be relied upon (by virtue of age or infirmity) to properly retain the thermometer for the necessary period of insertion in the body, may necessitate the physical presence of medical personnel during a relatively long measurement cycle, thus wasting valuable time. Furthermore, glass bulb thermometers are not as quick and easy to read and, hence, measurements are prone to human error, particularly when made under poor lighting conditions or read by harried personnel.
Various attempts have been made by the prior art to minimize or eliminate the aforedescribed deficiencies of the glass bulb thermometer by using appropriate temperature sensing probes which are designed to operate in conjunction with direct reading electrical thermometer instrumentation, typically employing an output galvanometer having an indicator needle moving along a calibrated scale. However, such probes and electrical thermometers have typically proven to be just as slow in making temperature measurements as glass bulb thermometers and, at best, output measurements have been only slightly easier to read.
Hence, those concerned with the development and use of thermometer apparatus in the medical field as well as measurement systems generally, have long recognized the need for improved temperature measuring devices which result in accurate, reliable, more rapidly obtained and easily read measurements. An electronic thermometer satisfying all of these requirements is disclosed in the aforementioned co-pending application Ser. No. 45,990. This electronic thermometer provides a temperature measurement output as a direct digital display and further employs a novel anticipation technique to provide an advance indication of the temperature at which a thermistor probe will finally stabilize. The anticipation technique used causes the state of the digital measuring means to be altered during a measurement cycle by a single value representing a fixed temperature differential, the magnitude of the correction value and the duration of the temperature measurement cycle being correlated as a function of the rate of change of the temperature being measured.
Basically, the electronic thermometer described in parent application Ser. No. 45,990 includes a temperature-responsive transducer in one arm of an electrical bridge network, the bridge including a balancing arm having a variable impedance, the impedance being selectively varied under the control of a digital counter indicating temperature, the counter being continually counted up by electrical impulses so long as the transducer temperature exceeds the temperature represented by the bridge balance impedance. The time period between successive impulses to the counter, i.e., the pulse period or pulse rate, is correlated with the time vs. temperature characteristic of the temperature responsive transducer to selectively alter bridge balance, and, hence, the state of the counter, so that the counter will rapidly count up to the anticipated temperature at which the transducer will finally stabilize. The latter is accomplished substantially sooner than actual stabilization of the transducer at its final temperature. The final temperature registered in the counter is appropriately indicated by a digital display unit connected to receive the counter output, the digital display providing an easily read output indication of temperature.
An electronic thermometer embodying the various features of the invention set forth in patent application Ser. No. 45,990 may include a thermistor as a temperatureresponsive transducer in the measurement arm of a Wheatstone bridge, the balancing arm of the bridge having a bank of parallel resistances, each resistance being selectively inserted into the bridge balancing arm under the control of its own switch, all of these switches being in turn controlled by various counting states registered in the digital counter indicating temperature, the counter being counted up by clock pulses which are gated on only when the thermistor temperature exceeds the equivalent temperature represented by the resistance in the bridge balancing arm. Since the thermistor approaches its final stable temperature asymptotically, the last increments of temperature change occur very slowly, whereas the major portion of the temperature change, in stabilizing the thermistor at the temperature of the environment, occurs relatively rapidly. In this regard, the time period between clock pulses gated to the counter is correlated with the rate of change of the thermistor temperature to anticipate the remaining temperature differential between the actual thermistor temperature and the final thermistor temperature, and to alter the balancing arm of the bridge accordingly so that the counter registers the anticipated final temperature long before the thermistor would normally actually stabilize at such a final temperature. This results in a much more rapid, yet accurate and reliable temperature measurement.
Correlation of the time period between clock pulses, or pulse rate, passing through the counter, with the temperature vs. time characteristic of the thermistor, and altering the state of balance of the bridge, may be accomplished in any of several ways. In one embodiment of the electronic thermometer set forth in application Ser. No. 45,990, an additional resistance shunts the bridge balancing arm so that the balancing arm and the counter are offset by the equivalent of a predetermined temperature differential, i.e., the counter is driven to a higher counter state than would ordinarily be dictated by the actual thermistor temperature, in order to compensate for the additional resistance shunting the balancing arm. Hence, the temperature indicated by the counter display leads the actual temperature of the thermistor by the predetermined temperature differential. It is readily ascertained empirically, for any given thermistor probe, how the rate of change of temperature varies with time, and the latter is correlated with the time period between pulses passed to the counter to determine when the actual thermistor temperature differs from its final stable temperature by the aforedescribed predetermined temperature differential between the counter state and the temperature represented by the bridge balancing arm resistance. In this connection, the pulse period for pulses incrementing the counter is monitored and, when the pulse period reaches the proper magnitude, the pulses to the counter are gated off to freeze the counter and its associated display at an indication representing the anticipated final temperature of the thermistor.
In another embodiment described in application Ser. No. 45,990, a specified time of measurement is selected, e.g., 15 seconds. At that point, voltage which is a function of the remaining temperature differential between the actual thermistor temperature and its ancitipated final temperature is inserted into the bridge balancing arm to deliberately unbalance the bridge and to force the counter to rapidly count up to the state representing the final anticipated temperature.
The temperature anticipation method described in connection with the aforementioned electronic thermometer system applies a fixed correction to the temperature measured by the thermistor probe, under the assumption that the heating of the probe occurs with essentially the same temperature vs. time characteristic each and every time a temperature is taken. However, variations in personnel measurement techniques, probe time constants, and even the variations in thermal response characteristic of biological tissue from one patient to another may cause variations in the final temperature vs. time function which affect the accuracy of the temperature readings obtained. In this regard, the use of a fixed correction is intended for an idealized case where substantially optimum technique is employed in taking temperatures.
For example, the optimum measurement technique may call for insertion of the thermistor probe into the patient's mouth and maintaining the probe tip in constant contact with the tissue under the tongue while sliding the probe tip along the tongue for five or six seconds to that the probe tip is continually exposed to fresh tissue during this time interval. Otherwise, a "draw down" phenomenon may occur wherein the probe tip cools down the tissue excessively so that the time constant for arriving at the final temperature measurement is different from the expected time constant for the idealized case. In addition, since counter pulse rate is monitored to either vary the magnitude of the correction or determine when the measurement cycle should be terminated, it will be apparent that loss of probe contact with the tissue early in the temperature measurement cycle may result in an unduly long pulse period, which causes the anticipation circuitry to believe it has reached the searched for portion of the time vs. temperature characteristic and, therefore, prematurely terminate the measurement cycle. In this case, a low temperature reading would result.
The aforedescribed requisites for optimum technique cannot reliably be obtained by untrained personnel, with a consequent requirement for relatively time-consuming and costly training effort necessary to insure proper usge by appropriate medical personnel.
Accordingly, those concerned with the development and use of temperature measurement methods and apparatus have recognized the desirability for further improvement in temperature measurement systems enabling enhanced accuracy, with even greater reliability, and with less dependence upon the use of optimum technique by personnel making such measurements. In addition, there has been a desire for improved electronic means for implementing such temperature measurement systems, characterized by greater accuracy, reliability, economy, simplicity, enhanced linearity of response, stability and suitability for implementation by modern electronic manufacturing methods, such as MOS (metal oxide semiconductor) technology. The present invention fulfills all of these needs.